The Sony Xperia M was launched back in 2013 and the device received official Android updates till version 4.3 (Jelly Bean). Does that mean it'll be stuck there forever? Well, no. The good news is that now you can experience Android Marshmallow on the device.
This is all thanks to alternative Android OS developer Cyanogen, which has released the Android 6.0.1 Marshmallow-based CyanogenMod 13 nightly for the handset. Before you go ahead with the install, make sure that your Xperia M device is running Android 4.3 Jelly Bean firmware.
The official nightly build is currently only available for Xperia M, meaning M Dual users are currently out of luck, although not completely as there are some patches available on XDA (head to the Source 2 link below) that'll help you experience Marshmallow on your device.
guys how to downgrade to 4.3 plz help my c2004 got bricked during this procedure
Yes, they have gotten way better recently, back in the X8 days (I had one) it was nonexistent, released with old software (and old Android on these days was way more important than nowadays), and one update to an outdated version and done. On the X...
Official cm13 for z3 is in the works so give them some time to polish things out ;-) don't worry, it will eventualy be avaliable.